1. Field of the Invention
This invention is related to a nitride semiconductor ultraviolet light-emitting device having high emission intensity.
2. Description of the Background Art
Nitride semiconductors such as AlGaN, GaN and InGaN are compound semiconductors that can emit light in a range from an ultraviolet region to an infrared region. In recent years, white LEDs (light-emitting diodes) are practically used for mobile phones and for backlight sources of liquid crystal televisions, and applications thereof are also growing for general illumination sources in place of incandescent lamps and fluorescent lamps. A white LED can be obtained by combining a fluorescence substance and a blue light-emitting diode formed with nitride semiconductor such as InGaN having an emission wavelength around 460 nm.
In a shorter emission wavelength range, there is also practically available a light-emitting diode including an active layer (emission layer) of InGaN-based nitride semiconductor having an emission wavelength in a range from ultraviolet longer than 365 nm to purple. However an ultraviolet light-emitting diode including an emission layer containing Al and having an emission wavelength shorter than 365 nm is still in research and development phases.
The AlGaN semiconductor is able to emit light in a range from ultraviolet of 365 nm wavelength to deep ultraviolet of 210 nm wavelength with adjustment of its Al composition ratio. Therefore it is hoped to use ultraviolet light-emitting diodes in place of the conventional ultraviolet lamps and conventional ultraviolet light sources and also hoped to apply them to the technical fields of sterilization, resin curing, medical treatment, etc. and thus research and development thereof are actively carried out at the present time.
In the meantime, in order to make shorter the emission wavelength of the ultraviolet light-emitting diode including the AlGaN emission layer, it is necessary to increase the Al composition ratio of the emission layer. Further, in order to suppress absorption of ultraviolet light in the semiconductor layers constituting the light-emitting diode, it is necessary to increase the Al composition ratios of the AlGaN layers in the n-type and p-type layer regions as compared to the emission layer. However in the case of increasing the Al composition ratio, not only the p-type layer but also the n-type layer becomes highly-resistive. As a result, electric current diffusion becomes insufficient in the semiconductor layers of the p-type and the n-type, and then the concentration of carriers injected into the emission layer becomes non-uniform whereby causing deterioration of the emission efficiency.
Further, since sapphire generally used as a substrate of the light-emitting device has low heat conductivity, there is a possibility that the temperature of the device increases due to insufficient heat dissipation during a high temperature operation or a high output operation of the device thereby causing saturation of the emission power or breakdown of the device.
Blue light-emitting diodes usually have their device structure of the lateral-type structure. More specifically, in the blue light-emitting device, the n-type semiconductor layer is partly exposed by dry etching, the n-type electrode is formed on the exposed part, and the transparent electrode of ITO (indium tin oxide) is fanned on the remaining p-type semiconductor layer part, so as to improve the efficiency of taking out light from the device. In general, even in the case of the blue light-emitting diode having the lateral-type structure, the n-type semiconductor layer has relatively low resistivity. Therefore if the contact electrode is formed in a wide area on the p-type semiconductor layer, electric current is sufficiently diffused in the p-type layer and uniformly injected into the emission layer whereby it is possible to obtain uniform light emission. However as a heat dissipation measure in the case of a high output LED and a large size LED requiring more uniform current injection or an LED required to operate at a high temperature, there is a case that the sapphire substrate for growth of the semiconductor layers is removed by laser lift-off and replaced with a conductive support substrate having high heat conductivity by a bonding method or the like so as to form the vertical-type structure and improve the properties of current diffusion, heat dissipation and the like.
In Patent Document 1 of Japanese Patent Laying-Open No. 2008-53425 and Patent Document 2 of Japanese Patent Laying-Open No. 2011-151393, in order to further improve the optical output of the vertical-type device in which the substrate is removed, there are disclosed a technique to provide a reflective electrode and a technique to provide a transparent electrode such as of ITO formed on the n-type electrode side where the substrate has been removed. Patent Document 3 of Japanese Patent Laying-Open No. 2006-278554 also discloses a technique to provide a transparent electrode of ITO in an ultraviolet LED having a peak emission wavelength around 330 nm Further, Patent Document 4 of Japanese Patent Laying-Open No. 2011-60917 describes that it is possible in a visible light LED to use Ga-doped MgZnO or Al-doped MgZnO for the transparent electrode formed on the p-layer side.
However it is not possible to directly apply the techniques of the blue light-emitting diodes as disclosed in Patent Documents 1-4 to the ultraviolet light-emitting device. While ITO is generally used for the transparent conductive film or transparent electrode in the blue light-emitting diode, ITO is not suitable for the transparent electrode material in the ultraviolet light-emitting device because the light absorption thereof becomes significant in the ultraviolet region. According to Patent Document 3, the transmittance of ITO is less than 70% for light of wavelength less than 320 nm, and thus it is not possible to use ITO as the transparent electrode for light of wavelength less than 320 nm. Further, even in the case that the ultraviolet light-emitting device is formed with the vertical-type simply having the flip-chip structure as disclosed in Patent Document 2, since both the layers of the p-type and the n-type are highly-resistive and electric current does not sufficiently diffuse, light emission is caused only in the vicinity of the electrode and then the emission efficiency cannot be improved. In order to diffuse the electric current, therefore, it becomes necessary that both the electrodes for the p-type and the n-type are formed as electrodes of wide areas covering almost the entire area from which light is taken out. Then, in order to efficiently take out light from the light-emitting device, it becomes necessary that at least one side electrode is made transparent for the emitted light.
The Ga-doped MgZnO or the Al-doped MgZnO for the transparent electrode in the visible-light LED as disclosed in Patent Document 4 has not sufficient conductivity and does not sufficiently function as the transparent electrode for the ultraviolet light-emitting device including the highly-resistive semiconductor layers. The reason for this is that since MgO is an insulator, MgZnO that is a compound thereof with ZnO is also highly-resistive and does not has such carrier mobility that generally enables it to function as the transparent conductive film.
As explained in the above, in the present circumstances, no electrode material has been found, which can sufficiently function as the transparent electrode for the nitride ultraviolet light-emitting device.